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Vincent Thacker

01:40

@Slereah They don't have to be zero at infinity, right?

There are clearly exact solutions to Einstein's equations that have gravity everywhere

4 hours later…

Manas Dogra

05:26

What is the form of momentum which is conserved in electrodynamics?
It isn't mv+qA the canonical momentum. Is it mv-qA as mentioned in my2cts answer[here](https://physics.stackexchange.com/questions/114466)

I doubt because the field contribution(volume integral of Poynting vector) isn't being accounted there..

2 hours later…

Slereah

07:46

@VincentThacker that's what I'm saying

@ManasDogra it's the Noether current, presumably

Depends on the form of the Lagrangian you're using

1 hour later…

satan 29

09:05

@ACuriousMind seems like my legacy in this room is eternal :P

Slereah

Well we all worship Satan here

satan 29

nice to hear.

2 hours later…

Slereah

11:02

there's so many questions on PSE because people confuse the Lagrangian and the action

I really wish universities tried better to explain it

sometimes the question is basically "How can there be a derivative if $f(0)$ is a constant value!"

3 hours later…

Ryan Unger

14:27

@Slereah hm

Nihar Karve

14:53

Is condensed matter a bit underrepresented on Physics.SE?

ACuriousMind

@NiharKarve You could say that, but I think it's more that hep-th is overrepresented

Slereah

Most sciences are underrepresented on PSE

There's almost no meteorology or metrology or geophysics or biophysics

it's mostly the "cool" sciences and the ones that are likely to be in undergrad physics classes

ACuriousMind

there's also Earth Science, Biology which get at least some questions that would also be on-topic here

and Astronomy, Quantum Computing, Matter Modeling

Slereah

Yeah I guess PSE is for more the "fundamental" disciplines?

I'm not sure I know anything about meteorology

ACuriousMind

there's also a founding effect where many of the initial members were hep-th theorists and so the site was seeded with such questions being overrepresented

Slereah

14:59

I think we did the basic model for atmospheric pressure and heat from the sun

that's about it

The earth is at static pressure and homogeneous temperature for me

it's a cool time to be a meteorologist I suppose

You get to portend doom

Nobody worries about thunderbolt singularities tho

What if the spacetime manifold just ends on tuesday???

Dynamically nothing forbids it

ACuriousMind

@Slereah at least I then don't have to laundry today

Nihar Karve

15:53

I would personally enjoy seeing a few more true gauge-theory, susy and solitons qft questions though

Slereah

16:08

there's plenty of those

although check out physics overflow if you want more maybe

Nihar Karve

well, only around 30 questions per year are asked across all three tags that I mentioned

Slereah

It's a lot for such a niche topic!

Nihar Karve

PO looks pretty dead these days tbh

(and I wouldn't be able to answer many of the questions anyway)

Faded Giant

17:12

a fun experimental riddle (solved):

Q: Peak in beer freezing temperature plot

I left an open beer in the freezer while monitoring its temperature. The first minutes works as expected. But after the plateau, at about −8 °C, a peak appeared: I thought it was an artefact because the can was open and the sensor was inside. So I repeated the experiment with an intact, closed c...

🍺

bolbteppa

18:03

There's no argument given for the gravitational field vanishing at infinity, it's just stated as an assumption. It doesn't seem to be used anywhere else in L&L and it's not even really used except for being framed as an essential difference between a non-inertial frame and a gravitational field, there's something subtle here at least

Slereah

or maybe he's wrong idk

Sometimes physicists put in weird notions that they have

bolbteppa

The comment is actually in the section on non-relativistic gravitational fields and not in the relativistic section, they don't give this argument in the relativistic section, maybe I am wrong thinking it also applies there, but even in the non-relativistic section, why is this an okay assumption if your model is just $L = mv^2/2 - m \phi$,

We can concoct solutions which don't go to zero at infinity right

Slereah

Non-relativistic may make sense, yes

Laplace equation with not zero at infinity value is Complicated

bolbteppa

It's Poisson with complicated boundary conditions at infinity, can you have solutions or are they impossible

If not then that is probably what's going on with the quote, if you can then maybe it's a physical assumption to exclude them because we are biased by Newton

Physics Meta

18:25

Q: Do we have a policy or guidance for questions with no one right answer?

This question does not seem to have a single correct answer posted. Which of the following are true: Assume that there will never be one right answer posted (i.e. because there is not in principle a right answer). This was a bad question and should be closed/deleted. Leave it how it is. It's ok...

discussion

Slereah

It's one of those issues physicists don't really care about anymore

it was of some concern in the 19th century but then GR happened a bit later so people kind of stopped caring

A few philosophers of physics talk about it and I don't remember if the conclusion is that it's "physical" or not

the rough idea being that if there's uniform matter distribution (so non-zero potential at infinity), then things diverge, and no matter what regularizing procedure you try to use, it will still not make sense in some way

bolbteppa

Poisson's equation

Poisson's equation is an elliptic partial differential equation of broad utility in theoretical physics. For example, the solution to Poisson's equation is the potential field caused by a given electric charge or mass density distribution; with the potential field known, one can then calculate electrostatic or gravitational (force) field. It is a generalization of Laplace's equation, which is also frequently seen in physics. The equation is named after French mathematician and physicist Siméon Denis Poisson. == Statement of the equation == Poisson's equation is Δ...

I think I remember what this gets into

bolbteppa

18:42

(I think) There's a boundary term missing in the Green function solution they give, maybe even in Newtonian GR this is just an assumption and one can ask why we can't assume it in GR too

Slereah

As I said, people tend to not be very fancy with Newtonian gravity today

Because it is the dumb child of GR